This application claims benefit of U.S. Provisional application Ser. No. 60/377,926, filed May 3, 2002, the entirety of the disclosure of which is incorporated herein by reference thereto.
This application relates generally to a method and equipment for weld overlay of metal tubing. More specifically, the invention relates to a system which employs a welding process to overlay conventional metal tubing in a single pass.
Boilers are used in the power generation, paper, steel, and waste-to-energy industries. These industries have an interest in finding means to protect and extend the operating cycles of their boilers. The operating cycles of the boiler are dependent on the fuel used by the boilers, methods of combustion, impact of environmental regulations, etc.
Boilers generally use tubing for carrying water which is heated to steam in the boiler. Such tubing often has structural requirements and must be corrosion and erosion resistant as well, in view of the high temperature and corrosive conditions within boilers.
In certain types of boilers, such as coal-fired, black liquor recovery, and waste-to-energy boilers, the requirement for corrosion and/or erosion resistance is heightened. In such applications, a solution is often found in applying one or more layers of alloy overlay material to the tubing having the desired corrosion and erosion resistant properties.
When boiler tubing is applied using a weld overlay process, the weld penetration of the tubing needs to be carefully controlled to ensure good fusion between a weld bead of overlay alloy material and tube stock, as well as between adjacent weld beads of the overlay material. The exterior surfaces of the completed weld overlay need to be smooth, and the thickness of the weld overlay needs to be uniform.
Conventional DC gas metal welding processes may produce excessive weld penetration, with what is known as “dilution” of the base metal into the weld metal in excess of 40%, when the weld overlay thickness is less then 0.030 inches thick. Weld overlays with dilution rates of this magnitude are not acceptable.
In the conventional application of weld overlay material, if the voltage and current of the weld head are controlled to ultimately provide a smooth exterior surface on the weld overlays, then weld penetration of the tube stock may tend to be significant and uneven, and this can result in excessive dilution.
On the other hand, if the voltage and current are controlled to provide optimum weld penetration, then the exterior surface of the weld overlay may tend to be rough and uneven. A rough surface finish provides a greater chance for corrosive media to become deposited on the overlay and for localized corrosion sites to form in the overlay. Further, rough exterior overlay surfaces can make the boiler tubes more difficult to bend, by causing an uneven distribution of the stresses to occur in the tubing during bending. Rough and uneven weld overlay surfaces can also affect the thickness and resulting dimensions of the tube.
Certain weld overlay methods have been patented. For example, U.S. Pat. No. 6,013,890, issued to Hulsizer, discloses a dual weld pass overlay method and apparatus which uses a first weld head to apply a bead of weld overlay material onto a tube, and thereby create a heat-affected zone in a tube. A second weld head is then used to heat the material within the heat-affected zone to “a temperature higher than its tempering temperature but lower than its Ae1 temperature.” U.S. Pat. No. 6,204,477 B1, issued to Lai also discloses a weld overlay method.
A need still exists, however, for a single pass weld overlay process which can provide uniform weld penetration on tube stock, uniform fusion of the overlay to the tube stock, smooth and even exterior surfaces on the overlay, and also, which can control the thickness of the overlay and, thus, the resulting dimensions of the tube once the overlay has been applied.